My control number in box d is just 4, but it's telling me that it must be two characters in order for me to file. Am I doing something wrong?

On a W-2, a control number in Box D is not required by the IRS.  It's for your employer/payroll department's purpose, or sometimes used when importing a W-2 from the employer/payroll department.   You can try leaving the Box D control number blank and trying to efile.

If it won't let you efile it with a blank in Box D, see this FAQ:

FAQ:  Box D Control Number on W-2

https://ttlc.intuit.com/turbotax-support/en-us/help-article/import-export-data-files/box-control-number-w-2/L8MD2FqqP_US_en_US